1. Field of the Invention
This invention relates to a power supply, an image forming apparatus, and an electric current supply method applied to them. More specifically, this invention relates to a power supply which supplies an electric current to a heating unit by using an induced heating method, an image forming apparatus which forms an image according to the electrophotographic method, and an electric current supply method applied to the power supply and image forming apparatus.
2. Description of the Related Art
An image forming apparatus which forms an image according to the electrophotographic method generally comprises a fixing unit for fixing a toner image transferred onto a printing medium. The fixing means often employs a heating method using a ceramic heater, halogen heater, or the like. Recently, the fixing means employs an electromagnetic induced heating method.
FIG. 12 is a block diagram showing the arrangement of a power supply which supplies power to a fixing unit using the induced heating method.
As shown in FIG. 12, a power supply 100 and fixing unit 7 are connected to each other by wires W1 and W2 via connectors A and B. The power supply 100 comprises a diode bridge 101, a filter capacitor 102, resonant capacitors 105 and 106 which form a resonant circuit, a coil L, and switching elements 103 and 104. The diode bridge 101 and the filter capacitor 102 form a circuit for supplying a direct current. The power supply 100 further comprises a driving unit 112 which drives the elements 103 and 104 respectively by driving signals D1 and D2, a control unit 113, a power detection unit 111 which detects power input from an AC power supply 500, and a temperature detection unit 114 which detects the temperature of an FB (Fixing Belt) serving as a conductive heating member.
The control unit 113 determines the driving frequencies of the driving signals D1 and D2 output from the driving unit 112 on the basis of the detection result by the power detection unit 111 and that by the temperature detection unit 114 (see, e.g., Japanese Patent Application Laid-Open No. 2000-223253). The switching elements 103 and 104 are alternately turned on/off in accordance with the driving signals D1 and D2 to supply a high-frequency electric current to the coil L.
Power control and temperature control of the fixing unit 7 are implemented using a change in power supplied to the fixing unit 7 in accordance with the driving frequencies of the driving signals D1 and D2.
However, according to the conventional induced heating method, the power factor is smaller than that of a ceramic heater or halogen heater. Power is a value calculated by multiplying the product of the effective values of an electric current and voltage by the power factor. To obtain a large heat amount, the electric current or voltage must be increased. However, connectors for supplying a large electric current are less stocked as components, are expensive, and have an excessively large size.
It is, therefore, desired to supply a large electric current while installing wiring with widespread connectors for a relatively small electric current.